1. Field of the Invention
The present invention relates to a hydrostatic continuously variable transmission, and more particularly to a hydrostatic continuously variable transmission for preventing an excessive increase in hydraulic pressure during speed reducing operations.
2. Description of the Background Art
A hydrostatic continuously variable transmission has been used for continuously variable transmissions for motorcycle and automobiles in the background art. Such a continuously variable transmission is described in Japanese Examined Patent Publication No. 7-56340 and Japanese Unexamined Patent Publication No. 4-203553. FIG. 6 is a schematic view of a hydrostatic continuously variable transmission according to the background art.
As shown in FIG. 6, this hydrostatic continuously variable transmission has a fixed displacement type swash plate hydraulic pump P connected to the crank shaft side of an engine and a variable displacement type swash plate hydraulic motor M connected to a driving wheel side of an engine. The hydraulic pump P and the hydraulic motor M are connected to each other to form a hydraulic closed circuit via an inside oil passage (low pressure passage) 52.
The inside oil passage 52 is a low pressure passage in a normal load operation, but is a high pressure passage in a speed reducing operation, or a reverse load operation. An outside oil passage (high pressure passage) 53 is a high pressure passage in a normal load operation, but is a low pressure passage in the reverse load operation. An oil supply passage 47 connected to an oil supply pump 88 pumping oil from an oil reservoir 87 is connected to the inside oil passage 52 via a first check valve 95 and is connected to the outside oil passage 53 via the second check valve 96.
A pressure regulating valve 97 is disposed between the outside oil passage 53 and the inside oil passage 52. In a normal operation, e.g. an acceleration operation, if the hydraulic pressure in the outside oil (high pressure) passage 53 is larger than a predetermined value, the pressure regulating valve 97 introduces part of the hydraulic oil in the outside oil passage 53 into the inside oil passage 52 to prevent an excessive pressure in the outside oil passage 53.
In a hydrostatic continuously variable transmission having the configuration described above, when the gear is operated in a reverse load, e.g. the engine acts as a brake, the gear is driven by the wheels. Accordingly, the hydraulic motor M acts as a hydraulic pump and the hydraulic pump acts as a hydraulic motor. The outside oil passage 53 becomes a low pressure oil passage and the inside oil passage 52 becomes a high pressure oil passage.
In particular, when a vehicle jumps and lands on the ground, the pressure in the inside oil passage 52 increases excessively. This excessive increase in the pressure in the inside oil passage 52 requires an increase in the size of the transmission.
The present invention overcomes the shortcomings associated with the background art and achieves other advantages not realized by the background art.
The present invention has been made in view of the realization that hydrostatic continuously variable transmissions of the background art can prevent an excessive increase in the hydraulic oil during normal operations, but are not protected against excessive pressures during speed reducing operations.
An aspect of the present invention is to prevent excessive pressure buildup in the low pressure side of a hydrostatic continuously variable transmission during all modes of operation.
These and other aspects of the present invention are accomplished by a hydrostatic continuously variable transmission comprising a hydraulic pump; a hydraulic motor; a high pressure oil passage disposed between the hydraulic pump and the hydraulic motor, the high pressure oil passage sending hydraulic oil to the hydraulic motor from the hydraulic pump during a normal operation of the transmission; a low pressure oil passage disposed between the hydraulic pump and the hydraulic motor, the low pressure oil passage sending hydraulic oil to the hydraulic pump from the hydraulic motor during the normal operation, and wherein the high and low pressure passages, the hydraulic pump and the hydraulic motor form a hydraulic closed circuit having a hydraulic pressure; and a pressure regulating valve for relieving the hydraulic pressure in the low pressure oil passage to the high pressure oil passage when the hydraulic pressure in the low pressure oil passage exceeds a predetermined value.
These and other aspects of the present invention are further accomplished by a hydrostatic continuously variable transmission comprising a hydraulic pump; a hydraulic motor; a cylinder block integrally formed by the hydraulic pump and the hydraulic motor; a hollow output shaft; a high pressure oil passage disposed between the hydraulic pump and the hydraulic motor, the high pressure oil passage sending hydraulic oil to the hydraulic motor from the hydraulic pump during a normal operation of the transmission; a low pressure oil passage disposed between the hydraulic pump and the hydraulic motor, the low pressure oil passage sending hydraulic oil to the hydraulic pump from the hydraulic motor during the normal operation, and wherein the high and low pressure passages, the hydraulic pump and the hydraulic motor form a hydraulic closed circuit having a hydraulic pressure; and a pressure regulating valve for relieving the hydraulic pressure in the low pressure oil passage to the high pressure oil passage when the hydraulic pressure in the low pressure oil passage exceeds a predetermined value.
According to the hydrostatic continuously variable transmission having the above-mentioned configuration, even when the hydraulic pressure in a hydraulic circuit increases when the hydraulic motor acts as a hydraulic pump, e.g. when a vehicle is decelerated and reverse loading is experienced, the pressure regulating valve is opened to introduce hydraulic oil in the low pressure oil passage to a high pressure oil passage. Therefore, an excessive increase in the low pressure oil passage is prevented without a need to increase the size and capacity of the oil passages.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.